The invention is based on a priority application EP04290730.3 which is hereby incorporated by reference.
The present invention relates to a method for controlling sleep mode in a dual mode terminal comprising an ad-hoc interface and a cellular network interface.
Ad-hoc networks are becoming more and more successful in the marketplace as indicated by the increasing usage of Bluetooth and WLAN networks. This success is due to several reasons, including their lack of infrastructure requirements as well as their suitability for several application scenarios like the communication between laptops and/or handheld devices.
It is also possible to use ad-hoc networks as an extension of the cellular air-interface, leading to improvements in cellular capacity as well as coverage. Additionally Load balancing between different cells can be provided by relaying individual calls to adjacent cells that otherwise could not be reached.
As a consequence, the integration of cellular (GSM/GPRS and UMTS, Beyond 3G, 4G) and ad-hoc (Bluetooth, WLAN, 4G) networks must be appropriately managed.
FIG. 1 describes a scenario for the integration of ad-hoc and cellular mobile networks. In this scenario, either one direct cellular link (1) or two links (2) and (3) can be used to connect a mobile terminal to the cellular radio communication network. Link (2) connects the mobile terminal to a relay and link (3) connects the relay to the cellular radio communication network.
For working, this scenario requires following information:                which potential relays are available?        what is the quality or link performance of each link (cellular as well as ad-hoc links) that potentially could be used?        
The answers to these questions are required to make a sound decision between the different alternatives including the direct cellular link
An issue to solve in such a dual mode mobile terminal comprising an ad-hoc interface and a cellular network interface is the power consumption. In ad-hoc as well as in cellular technologies a sleep mode is provided at the mobile terminal to save battery power. The mobile terminal wakes up periodically from the sleep mode in order to listen to incoming calls. The less often the terminals need to wake-up the longer the sleep mode could be maintained and the longer the battery power would suffice. Hence, there is a trade-off between idle mode power consumption and reaction time.
Since battery power is especially very scarce in mobile terminals communicating over ad-hoc networks, elaborated sleep modes have been designed to save battery power (see background at the end of this text). Depending on the exact type, situation and configuration of sleep mode, the waking up can take some few 100 ms up to several seconds.
In the following, conventional sleep mode and wake-up mechanisms are described for cellular radio communication networks as well as for ad-hoc networks.
In cellular networks (example UMTS), the Paging Channel (PCH) is operated together with the Paging Indicator Channel (PICH) to provide terminals with sleep mode operations. The Paging Indicators (PI) occur once per slot on the corresponding physical channel, the PICH. Depending on the paging indicator repetition ratio, there can be 18, 36, 72 or 144 Paging Indicators per PICH frame. How often a terminal needs to listen to the PICH is parameterized, and the exact moment depends on the system frame number (SFN). For detection of the PICH the terminal needs to obtain the phase reference from the Common Pilot Channel (CPICH).
A terminal, once registered to a network, has been allocated to a paging group. For the paging group there are Paging Indicators (PI), which appear periodically on the PICH when there are messages for any of the terminals belonging to the paging group. Once a PI has been detected, the terminal decodes the next PCH frame to see whether there was a paging message intended for it.
The less often the PI appear, the less often the terminal needs to wake-up from sleep mode and the longer the battery life becomes. The trade-off is the reaction time.
Within the IEEE 802.11 standard (WLAN based ad-hoc networks) the general idea is that all stations in power save mode should be synchronized to wake-up in the same time. At this time there starts a window in which the sender announces the buffered frames for the receiver. A station that receives such an announcement frame stays awake until the frame is delivered.
In infrastructure mode, the access point is able to store packets for stations in doze state and to synchronize all mobile stations. In ad-hoc mode, the packet store and timing synchronization has to be done in a distributed manner. Power saving in IEEE 802.11 consists of a Timing Synchronization Function (TSF) and the actual power saving mechanism.
In the case of TSF for infrastructure mode, the access point is generating beacons, which contain a time stamp. Stations adjust their local timers to that time stamp. Beacons are sent periodically in beacon periods. Doze stations have to wake-up once in one beacon interval. If the channel is in use after the beacon interval the access point has to defer its transmission until the channel is free again.
In ad-hoc mode the timers adjust in a distributed way: every station is generating a beacon. After the beacon interval all stations compete for transmission of the beacon using the standard back-off algorithm. The first station wins the competition and all other have to cancel their beacon transmission and to adjust their local timers to the time stamp of the winning beacon. The stations should wake-up at the beginning of each beacon interval. The period in which one station is awake consists of a deferral delay (if the channel is in use after the previous beacon interval), a random time interval till the beacon is sent and another random time interval after the beacon is sent.
The power management is based on the distributed scheme as it is used for TSF. Packets for a station in doze state have to be buffered by the sender until the end of the beacon interval. The station in doze mode is first announced during the period when all the stations are awake. The announcement is done via an ad-hoc traffic indication message (ATIM), which is transmitted in a special interval (the ATIM window) directly after the beacon. A station in the power save mode should listen for these announcements to determine if it needs to remain in the doze state.
In Bluetooth based ad-hoc networks, there are synchronized sleep mode procedures, which can be only applied after the piconet has been established, and unsynchronized sleep mode procedures.
In the case of synchronized procedures there is little uncertainty in time and frequency. The slave can be “parked” by the master, similar to how cellular terminals are locked to their radio access node. Therefore for these procedures the sleep time can be longer.
In the case of unsynchronized procedures the unit that wants to connect has to solve the time-frequency uncertainty: it does not know when the idle unit will wake-up and on which frequency. For this, three mechanisms have been defined: inquiry, scan and page, which will be shortly explained in the next paragraphs.
In order to determine the page message and the wake-up sequence, the paging unit must know the identity of the unit in idle mode. If this information is not known, the paging unit may broadcast an inquiry message that induces recipients to return their identity and clock information.
To further describe paging and scan procedures it is assumed that the paging unit knows the identity of the unit in idle mode, but it does not know the clock information.
The unit in idle mode periodically wakes-up to listen for paging messages containing its identity. Every time the unit wakes-up it scans a different hop carrier (for interference reasons). When a Bluetooth unit wakes up to scan, it opens its sliding correlator, which is matched to the access code derived from its own identity. The scan window is 10 ms and the wake-up sequence is 32 hops in length and cyclic. The phase of the sequence is derived from the native clock of the unit.
If the paging unit knows the identity of the unit in sleep mode, then it knows the wake-up sequence and also generates the access code, which serves as paging message. The paging unit transmits the access code repeatedly at different frequencies every 1.25 ms; the paging unit transmits two access codes and listens twice for an answer. Consecutive access codes are transmitted on different hops selected from the wake-up sequence. In a 10 ms period 16 different hop frequencies are visited, which represent half of the wake-up sequence. The paging unit transmits the access code on these 16 frequencies cyclically for the duration of the sleep period of the idle unit.
If the idle unit wakes-up in any of these 16 frequencies, it will receive the access code. However, since the paging unit does not know the phase the idle unit is using, the idle unit can equally well wake-up in any of the 16 remaining frequencies from the 32-hop wake-up sequence. Therefore, if the paging unit does not receive a response after a time corresponding to the sleep time, it will transmit the access code repeatedly on the hop carriers from the remaining half of the sequence. The maximum access delay therefore amounts to twice the sleep time.
If the paging unit knows the clock of the idle unit then the average response time is reduced to half of the sleep time.
A mobile terminal comprising a first cellular interface and a second ad-hoc interface should wake-up periodically on the cellular as well as on the ad-hoc air interface.
A drawback is that the power consumption is heavily increased (roughly doubled) compared to a single air interface, leading to shorter battery life in sleep mode.
A particular object of the present invention is to provide a method for maintaining an efficient sleep mode for mobile terminals supporting two air interfaces: cellular and ad-hoc.
Another object of the invention is to provide a mobile terminal as well as a radio access node of the cellular network for performing this method.